As a production method of phosphoric acid, there have been known a wet method and a dry method, and phosphoric acid is industrially produced by one of these methods. In the wet method, phosphorous ore is dissolved in sulfuric acid, a gypsum component is filtered off to produce dilute phosphoric acid having a low concentration, and the dilute phosphoric acid is concentrated to a desired level to give phosphoric acid. In the dry method, phosphorous ore is reduced in an electric furnace to give yellow phosphorus, which is burnt to give phosphoric anhydride, which is then hydrated to give phosphoric acid. In general, the wet method does not require reduction or combustion and is considered to be advantageous from the aspect of production cost. The dry method is considered to be advantageous from the aspect of quality, because the production via phosphoric anhydride reduces contamination of impurities derived from phosphorous ore. In any production method, however, the obtained phosphoric acid contains arsenic derived from phosphorous ore or phosphoric anhydride, which is harmful for human body, in generally about several dozen ppm, and the arsenic is removed by a sulfide coagulation method according to the use of the phosphoric acid. A conventional method for removing arsenic is applicable only to phosphoric acid having a P2O5 concentration of up to about 60%.
High concentration phosphoric acid, particularly polyphosphoric acid (P2O5 concentration of not less than 72.4%), is also produced by the aforementioned wet method or dry method, like phosphoric acid. Polyphosphoric acid produced by the dry method generally shows lower heavy metal, silica and sodium contents because the production goes through phosphoric anhydride. However, it contains arsenic derived from phosphoric anhydride in about 5-100 ppm, because a conventional method for removing arsenic cannot be applied to high concentration phosphoric acid. Polyphosphoric acid produced by the wet method shows a low arsenic content of less than 1 ppm because a conventional method for removing arsenic can be applied before concentration. On the other hand, it characteristically shows high contents of heavy metal, silica and sodium derived from phosphorous ore. High concentration phosphoric acid, particularly polyphosphoric acid, which characteristically shows a low arsenic content and low contents of heavy metal, silica and sodium, has not been obtained.
When phosphoric acid having a P2O5 concentration of up to about 60%, which is obtained by a dry method, is subjected to a dearsenic treatment by a conventional sulfide coagulation method and the like and concentrated, polyphosphoric acid having low contents of arsenic, heavy metal, silica and sodium is presumably obtained. However, such production method has not been put to practical use. This is because equipment for burning and hydrating yellow phosphorus and for highly concentrating the resulting phosphoric acid is required, which is extremely disadvantageous from the economical point of view, due to high facility costs and high operation costs.
The arsenic contained in phosphoric acid and polyphosphoric acid is particularly problematic for a use that requires high purity, in the fields of food, medicaments, electronic materials and the like. Given the high attention to the environmental problems in recent years, moreover, phosphoric acid and polyphosphoric acid having a low arsenic content are desired as industrial phosphoric acid to be used for metal surface treatment, dye processing and the like. Therefore, a method for removing arsenic, which affords efficient operation at lower costs, is demanded.
Known methods for removing arsenic from phosphoric acid include (1) sulfide coagulation method, (2) solvent extraction method and (3) ion exchange method. Of these, the sulfide coagulation method is most popular because the required steps and equipment are rather simple and this method is operable at a low cost. This method includes bringing phosphoric acid into contact with hydrogen sulfide, or sodium sulfide, sodium hydrosulfide and the like, which produce hydrogen sulfide upon dissolution in phosphoric acid, to allow precipitation of arsenic as arsenic sulfide, which is followed by separation for removal. In contrast, the solvent extraction method and ion exchange method are less popular because of the complicated steps and complicated equipment, as well as higher costs.
However, the sulfide coagulation method, too, requires filtration equipment, such as vacuum filter, press filter, centrifuge separator and the like. The precipitated arsenic sulfide tends to become a gel, which makes it difficult to separate and remove arsenic from phosphoric acid. For facilitated separation, a method (Japanese Unexamined Publication No. 6-48712) comprising passage through an activated carbon tower, and a method (Japanese Unexamined Publication No. 6-100307) comprising addition of a chelating resin are used. These methods are unpreferable because the facility and treatment costs become higher.
When hydrogen sulfide remains in phosphoric acid, moreover, corrosion of phosphoric acid is enhanced, thus necessitating deaeration of excess hydrogen sulfide with air or nitrogen, which in turn makes the step complicated. When compounds such as sodium sulfide, sodium hydrosulfide and the like are used, this method cannot be applied to a use where a sodium content matters, since sodium remains in a large amount in phosphoric acid.
In the case of the sulfide coagulation method, when the phosphoric acid concentration is high, particularly in the case of polyphosphoric acid, phosphoric acid itself comes to have a higher viscosity, thereby making the separation for removal of arsenic sulfide more difficult. Therefore, this method is applicable to phosphoric acid having a P2O5 concentration of about 60% at most.
The method for removing arsenic from phosphoric acid having a higher concentration is disclosed in Japanese Examined Publication No. 53-128595 as the purification method of phosphoric acid by a wet method. According to this method, sodium chloride is added to phosphoric acid having a P2O5 concentration of up to about 82% to allow reaction of arsenic in phosphoric acid with chlorine for separation and removal. Because this method involves the use of sodium chloride in 1000-2000 ppm, sodium remains in a great amount in phosphoric acid after removing arsenic.
The conventional methods for removing arsenic require a filtration step and a deaeration step, both of which are costly, and sodium remains in a great amount in phosphoric acid. They cannot be applied to high concentration phosphoric acid having a P2O5 concentration of not less than 60%, particularly polyphosphoric acid. In addition, there has not been obtained high concentration phosphoric acid, particularly polyphosphoric acid, having lower contents of arsenic, heavy metal, silica and sodium, by a method operable at a low cost. Therefore, the field of use of polyphosphoric acid has been limited to the field free of a high purity requirement.
The present invention provides a completely new method for removing arsenic from phosphoric acid, which is operable at a low cost, which requires no filtration step or deaeration step but is free of a large amount of residual sodium, unlike sulfide coagulation method, and which is applicable to high concentration phosphoric acid, particularly polyphosphoric acid. In addition, the present invention aims at obtaining high purity polyphosphoric acid having low contents of arsenic, heavy metal, silica and sodium, which has not been conventionally obtained.